Methods of treating metal carbonate salts

ABSTRACT

A method of treating a metal carbonate salt includes hydrolyzing a metal halide salt to form a hydrohalic acid and a hydroxide salt of the metal in the metal halide salt. The metal includes an alkaline earth metal or an alkali metal. The method includes reacting the hydrohalic acid with the metal carbonate salt, wherein the metal carbonate salt is a carbonate salt of the alkaline earth metal or alkali metal, to form CO2 and the metal halide salt. At least some of the metal halide salt formed from the reacting of the hydrohalic acid with the metal carbonate salt is recycled as at least some of the metal halide salt in the hydrolyzing of the metal halide salt to form the hydrohalic acid and the hydroxide salt.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 63/200,390 filed Mar. 4, 2021, thedisclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

Despite the high regeneration energy requirements for hydroxide-basedsolvents like KOH, CO₂ direct air capture (DAC) systems based on it arebeing actively commercialized primarily because of the implementationadvantages of solvents over DAC alternatives such as solid sorbents andCO₂ selective membranes. These latter alternatives can have lowerspecific energy requirements compared to calcination-based regeneration,but only solvent capture currently appears advanced enough tosuccessfully address the engineering challenges associated withlarge-scale air contactor design and function.

Solvent DAC systems are built around simple liquid-gas contactors forCO₂ absorption and centralized facilities for solvent regeneration.These systems can be robust since the active material (e.g., the liquidcapture solvent), can be continually reconditioned while circulating,without having to pause CO₂ capture for a regeneration cycle. Basicliquid—air contactor models exist in the form of cost-effective coolingtowers that are already implemented at the scales being discussed forglobal DAC impact. Also, by centrally regenerating the CO₂-rich solventinstead of distributing that function throughout the air contactor, itis possible for solvent regeneration to take advantage of equipmenteconomies of scale, in contrast to sorbents and membranes which scalelinearly through the addition of multiple units.

The active DAC component of sorbent-based systems, in contrast, isgenerally immobile, and provisions for capture, regeneration, and CO₂desorption must be incorporated into the air contactor itself.Therefore, the advantage of a sorbent system's lower regenerationtemperature compared to a solvent would seem to be offset by the need tocyclically distribute low-grade thermal energy throughout a largeair-contacting structure.

Membrane-based DAC has a similar area-related drawback in that theactive element, i.e., the membrane, serves as the air contactorinterface, and a driving pressure gradient (with one side likely undervacuum) must be maintained across the entire surface. Given the smallCO₂ pressure gradient that is available in the atmosphere, leaksanywhere throughout the system greatly degrade DAC performance.

The world's oceans are estimated to have absorbed roughly one-third ofthe CO₂ added to the atmosphere from human activities, lowering its pHby 0.1 units in the process. This accumulation affects the health of theoceans since CO₂ acidification contributes to coral bleaching and ithinders the growth of shell-forming marine animals. As such, large-scaleocean CO₂ removal using lime addition has been identified as a potentialtool to improve the health of the oceans and to also possibly assistwith moderating atmospheric CO₂ levels. However, the concept was basedon the one-time consumption of lime to sequester ocean CO₂ as limestone,CaCO₃, which would require a significant lime production source.

In another approach to ocean CO₂ removal, NaCl is electrochemicallysplit to form HCl and NaOH, and the HCl is used to lower the pH ofseawater, thereby converting bicarbonate and driving off CO₂ gas. TheNaOH is used to return pH to a normal range. Although bipolar membraneelectrodialysis (EDBM) had advantages for producing HCl and NaOH fromNaCl regarding feed water purification compared to chlor-alkalielectrolysis and has a lower theoretical minimum energy requirement, interms relative to CO₂ removal, the energy consumption of state of theart EDBM is unacceptably high; the minimum energy requirement is 2.3kWe/kg to produce weakly concentrated NaOH. Based on this value ofelectricity consumption and assuming a 1:1 molar utilization of NaOH toCO₂ gas, the EDBM approach to CO₂ removal would require at least +331kJ/mol CO₂ of energy input; at $60/MWhe, the energy cost alone wouldexceed $114/ton CO₂. Improvements with or alternatives to NaClsalt-splitting technology are needed to achieve cost targets below$100/ton CO₂.

SUMMARY OF THE INVENTION

A method of treating a metal carbonate salt includes hydrolyzing a metalhalide salt to form a hydrohalic acid and a hydroxide salt of the metalin the metal halide salt. The metal includes an alkaline earth metal oran alkali metal. The method also includes reacting the hydrohalic acidwith the metal carbonate salt, wherein the metal carbonate salt is acarbonate salt of the alkaline earth metal or alkali metal, to form CO₂and the metal halide salt. At least some of the metal halide salt formedfrom the reacting of the hydrohalic acid with the metal carbonate saltis recycled as at least some of the metal halide salt in the hydrolyzingof the metal halide salt to form the hydrohalic acid and the hydroxidesalt.

A method of treating CaCO₃ includes hydrolyzing CaCl₂ to form HCl andCa(OH)₂. The method also includes reacting the HCl with the CaCO₃, toform CO₂ and CaCl₂, wherein at least some of the CaC12 formed from thereacting of the HCl with the CaCO₃ is recycled as at least some of theCaC12 in the hydrolyzing of the CaCl₂ to form the HCl and the Ca(OH)₂.

A method of regenerating a used hydroxide-based CO₂-capture sorbentincludes hydrolyzing a metal halide salt to form a hydrohalic acid and ahydroxide salt of the metal in the metal halide salt. The metal includesan alkaline earth metal or an alkali metal. The method includes reactingthe used hydroxide-based CO₂-capture sorbent with the hydroxide salt, toform a carbonate salt of the metal in the metal halide salt. The methodalso includes reacting the hydrohalic acid with the carbonate salt, toform CO₂ and the metal halide salt. At least some of the metal halidesalt formed from the reacting of the hydrohalic acid with the carbonatesalt is recycled as at least some of the metal halide salt in thehydrolyzing of the metal halide salt to form the hydrohalic acid and thehydroxide salt.

A method of regenerating a used hydroxide-based CO₂-capture sorbentincludes hydrolyzing CaCl₂ to form HCl and Ca(OH)₂. The method includesreacting the used hydroxide-based CO₂-capture sorbent with the Ca(OH)₂,to form CaCO₃. The method also includes reacting the HCl with the CaCO₃,to form CO₂ and CaCl₂. At least some of the CaCl₂ formed from thereacting of the HCl with the CaCO₃ is recycled as at least some of theCaCl₂ in the hydrolyzing of the CaCl₂ to form the HCl and the Ca(OH)₂.

A method of softening water includes hydrolyzing a metal halide salt toform a hydrohalic acid and a hydroxide salt of the metal in the metalhalide salt. The metal includes an alkaline earth metal or an alkalimetal. The method includes reacting a bicarbonate salt from a watersource with the hydroxide salt, to form a carbonate salt of the metal inthe metal halide salt. The method also includes reacting the hydrohalicacid with the carbonate salt, to form CO₂ and the metal halide salt. Atleast some of the metal halide salt formed from the reacting of thehydrohalic acid with the carbonate salt is recycled as at least some ofthe metal halide salt in the hydrolyzing of the metal halide salt toform the hydrohalic acid and the hydroxide salt.

A method of softening water includes hydrolyzing CaCl₂ to form HCl andCa(OH)₂. The method includes reacting Ca(HCO₃)₂ from a water source withthe Ca(OH)₂, to form CaCO₃. The method also includes reacting the HClwith the CaCO₃, to form CO₂ and CaCl₂. At least some of the CaCl₂ formedfrom the reacting of the HCl with the CaCO₃ is recycled as at least someof the CaCl₂ in the hydrolyzing of the CaCl₂ to form the HCl and theCa(OH)₂.

Various aspects of the methods of the present invention have advantagesover other methods. For example, various aspects of the method oftreating a metal carbonate salt can be used as an alternative to thecalcination of limestone for the production of hydrated lime. The formedhydrohalic acid can be used to decompose natural limestone, resulting ina metal halide salt solution that can be hydrolyzed to form hydratedlime. An advantage of this process compared to conventional limestonecalcination is the use of less energy; brine hydrolysis could lower theheat source temperature from 900° C. to 400° C. or lower. Also, byincorporating the exothermic lime hydration reaction into the hydrolysisstep (i.e., CaO to Ca(OH)₂), the minimum required thermal energy can belowered by 28%, from 178 kJ/mol CaCO₃ for calcination to 128 kJ/molCaCO₃ for the present invention. Additionally, the CO₂ released fromlimestone decomposition can be isolated (e.g., from flue gases) andreadily captured for sequestration. The cement production industry canbenefit from aspects of the method of treating a metal carbonate salt,as this energy-intensive industry struggles to efficiently decarbonize.

Hydroxide-based CO₂-capture sorbents such as KOH and NaOH have desirableproperties when it comes to DAC; e.g., they can be used in large,scalable air contactors and can capture CO₂ continuously without pausingair flow for regeneration. However, a feature that makes these solventsattractive for DAC, their high affinity for CO₂, also makes them costlyto regenerate, various aspects of the present invention address thatdeficiency. In particular, existing solvent regeneration schemes usingCa causticization use the direct calcination (thermal decomposition) ofCaCO₃ to release the CO₂ product and regenerate the CaO capturematerial. This process occurs at high temperatures (900° C. or greater)and has a significant endothermic heat of reaction of approximately 178kJ/mol CO₂. Various aspects of the present method that use hydrolyzedCaCl₂ salt as a reaction intermediary can lower the required temperatureto below 500° C., which can expand the types of heat resources suitablefor regeneration. In various aspects, by incorporating Ca(OH)₂ formationinto the overall reaction, the theoretical input energy requirements canbe decreased, such as brought down to approximately 113 kJ/mol CO₂,since the hydroxide formation is exothermic. Conventional calcinationproduces CaO solids that must be separately slaked with water to produceCa(OH)₂. Low grade heat is released by lime slaking and can be recoveredfor drying, etc., but this energy cannot be used to offset thecalciner's high temperature energy demands.

In various aspects, the method of regenerating a used CO₂-sorbent canrequire a lower amount of energy than other methods of regenerating usedCO₂-sorbents, making direct air capture for removal of CO₂ a more viablecarbon management tool. In various aspects, the method of regenerating aused CO₂-sorbent can expand the feasible use of hydroxide solvents suchas KOH by reducing the temperature and quantity of regeneration energy.Hydroxide solvents have strong chemisorption capacity for CO₂, makingthem effective even with the low CO₂ partial pressure in the atmosphere,and have been the preferred choice for CO₂ DAC studies and pilot testsover amines that are considered the benchmark for postcombustion CO₂capture. Regeneration energy has been the primary drawback of hydroxidesolvents, and the lower-temperature, lower-energy regeneration methodsof the present invention can make them a superior choice for DACcompared to materials that operate at higher CO₂ concentrations.

In various aspects, the method of the present invention of removing CO₂from water can recycle the metal and halide constituents, as contrastedwith an electrochemical NaCl-based process where the split NaOH and HClconstituents are released with the treated seawater and replacement NaClbrine must be reconcentrated. In various aspects, the brine hydrolysisof the present invention can be more robust (e.g., can be more tolerantof other dissolved species found in seawater) and lower cost to operate.Excessive energy consumption hinders ocean CO₂ removal by increasingoperating costs and creating additional CO₂ emissions that need to beoffset. The method of the present invention of removing CO₂ from watercan provide a thermochemical cycle to achieve production of CO₂ frombicarbonate while enabling a lower cost of energy consumption comparedto what is possible today, making the concept of ocean CO₂ removal amore feasible tool for carbon management. Compared to a NaCl-basedprocess, the hydrolytic softening method described herein can be lessdisruptive to ocean life since it does not acidify the water which couldharm sensitive organisms. As a result, the hydrolytic softening of thepresent invention can present a relatively lower environmental risk andshould face fewer restrictions on its application.

In various aspects, methods of the present invention can use commoditymaterials, which can be an advantage over other methods that requireproprietary, tailored materials that may introduce bottlenecks to futurelarge-scale deployment.

BRIEF DESCRIPTION OF THE FIGURES

The drawings illustrate generally, by way of example, but not by way oflimitation, various aspects of the present invention.

FIG. 1 illustrates a process comparison between calcination and brinehydrolysis regeneration, in accordance with various aspects.

FIG. 2 illustrates pressure versus concentration for CaCl₂ brinehydrolysis at 400° C., in accordance with various aspects.

FIG. 3 illustrates a direct air capture process, in accordance withvarious aspects.

FIG. 4 illustrates a process for ocean CO₂ removal using hydrolyticsoftening, in accordance with various aspects.

FIG. 5 illustrates a cross-section of a reactor for decomposition ofCaCO₃, in accordance with various aspects.

FIG. 6 illustrates an apparatus for performing hydrolytic softening ofwater for carbon dioxide removal, in accordance with various aspects.

FIG. 7 illustrates an Arrhenius plot of CaCl₂ hydrolysis data, inaccordance with various aspects.

FIG. 8 illustrates percentage removal of CO₂ from seawater havingvarious pH levels, in accordance with various aspects.

FIG. 9 illustrates calculated saturation index values for CaCO₃ andMg(OH)₂ in seawater as a function of pH adjustment, in accordance withvarious aspects.

FIG. 10 illustrates the ratio of Ca removed to Ca added as Ca(OH)₂ forseawater having various pH levels, illustrating the experimentalrecovery ratio of calcium from seawater, in accordance with variousaspects.

FIG. 11 illustrates process flow values used for a techno-economicassessment, in accordance with various aspects.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to certain aspects of the disclosedsubject matter. While the disclosed subject matter will be described inconjunction with the enumerated claims, it will be understood that theexemplified subject matter is not intended to limit the claims to thedisclosed subject matter.

Throughout this document, values expressed in a range format should beinterpreted in a flexible manner to include not only the numericalvalues explicitly recited as the limits of the range, but also toinclude all the individual numerical values or sub-ranges encompassedwithin that range as if each numerical value and sub-range is explicitlyrecited. For example, a range of “about 0.1% to about 5%” or “about 0.1%to 5%” should be interpreted to include not just about 0.1% to about 5%,but also the individual values (e.g., 1%, 2%, 3%, and 4%) and thesub-ranges (e.g., 0.1% to 0.5%, 1.1% to 2.2%, 3.3% to 4.4%) within theindicated range. The statement “about X to Y” has the same meaning as“about X to about Y,” unless indicated otherwise. Likewise, thestatement “about X, Y, or about Z” has the same meaning as “about X,about Y, or about Z,” unless indicated otherwise.

In this document, the terms “a,” “an,” or “the” are used to include oneor more than one unless the context clearly dictates otherwise. The term“or” is used to refer to a nonexclusive “or” unless otherwise indicated.The statement “at least one of A and B” or “at least one of A or B” hasthe same meaning as “A, B, or A and B.” In addition, it is to beunderstood that the phraseology or terminology employed herein, and nototherwise defined, is for the purpose of description only and not oflimitation. Any use of section headings is intended to aid reading ofthe document and is not to be interpreted as limiting; information thatis relevant to a section heading may occur within or outside of thatparticular section.

In the methods described herein, the acts can be carried out in anyorder without departing from the principles of the invention, exceptwhen a temporal or operational sequence is explicitly recited.Furthermore, specified acts can be carried out concurrently unlessexplicit claim language recites that they be carried out separately. Forexample, a claimed act of doing X and a claimed act of doing Y can beconducted simultaneously within a single operation, and the resultingprocess will fall within the literal scope of the claimed process.

The term “about” as used herein can allow for a degree of variability ina value or range, for example, within 10%, within 5%, or within 1% of astated value or of a stated limit of a range, and includes the exactstated value or range.

The term “substantially” as used herein refers to a majority of, ormostly, as in at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%,98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more, or100%. The term “substantially free of” as used herein can mean havingnone or having a trivial amount of, such that the amount of materialpresent does not affect the material properties of the compositionincluding the material, such that about 0 wt % to about 5 wt % of thecomposition is the material, or about 0 wt % to about 1 wt %, or about 5wt % or less, or less than, equal to, or greater than about 4.5 wt %, 4,3.5, 3, 2.5, 2, 1.5, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1,0.01, or about 0.001 wt % or less, or about 0 wt %.

Method of Treating a Metal Carbonate Salt

Various aspects of the present invention provide a method of treating ametal carbonate salt. The method can include hydrolyzing a metal halidesalt to form a hydrohalic acid and a hydroxide salt of the metal in themetal halide salt. The metal can include an alkaline earth metal or analkali metal. The method can include reacting the hydrohalic acid withthe metal carbonate salt, wherein the metal carbonate salt is acarbonate salt of the alkaline earth metal or alkali metal, to form CO₂and the metal halide salt. At least some of the metal halide salt formedfrom the reacting of the hydrohalic acid with the metal carbonate saltcan be recycled as at least some of the metal halide salt in thehydrolyzing of the metal halide salt to form the hydrohalic acid and thehydroxide salt.

The metal carbonate salt can be any suitable metal carbonate salt. Insome examples, the metal carbonate salt is BeCO₃, MgCO₃, CaCO₃, SrCO₃,BaCO₃, RaCO₃, Li₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃, Cs₂CO₃, Fr₂CO₃, or acombination thereof. The metal carbonate salt can be CaCO₃, MgCO₃, or acombination thereof. The metal carbonate salt can be CaCO₃.

The metal carbonate salt can be from any suitable source, such as from asorbent, a water source (e.g., salt water or fresh water), or acombination thereof. The metal carbonate salt can be CaCO₃ and the CaCO₃can be produced from a CO₂-capture sorbent, is a CaCO₃ precipitateformed from water softening, is natural limestone (e.g., as used in thecement industry, or another industry), or a combination thereof.

The method can include hydrolyzing a metal halide salt to form ahydrohalic acid and a hydroxide salt of the metal in the metal halidesalt. The metal can include an alkaline earth metal or an alkali metal,such as beryllium, magnesium, calcium, strontium, barium, radium,lithium, sodium, potassium, rubidium, cesium, francium, or a combinationthereof. The alkaline earth metal or alkali metal can be magnesium,calcium, or a combination thereof. The alkaline earth metal or alkalimetal can be calcium.

The metal halide salt can be beryllium halide salt, a magnesium halidesalt, a calcium halide salt, a strontium halide salt, a barium halidesalt, a radium halide salt, a lithium halide salt, a sodium halide salt,a potassium halide salt, a rubidium halide salt, a cesium halide salt, afrancium halide salt, or a combination thereof. The halide can bechloride and the metal halide salt can be beryllium chloride, magnesiumchloride, calcium chloride, strontium chloride, barium chloride, radiumchloride, lithium chloride, sodium chloride, potassium chloride,rubidium chloride, cesium chloride, francium chloride, or a combinationthereof. The metal halide salt can be CaCl₂, MgCl₂, or a combinationthereof. The metal halide salt can be CaCl₂. The hydrohalic acid can beHCl, HBr, HI, HF, or a combination thereof. The hydrohalic acid can beHCl. The hydroxide salt can be Be(OH)₂, Mg(OH)₂, Ca(OH)₂, Sr(OH)₂,Ba(OH)₂, Ra(OH)₂, LiOH, NaOH, KOH, RbOH, CsOH, FrOH, or a combinationthereof. The hydroxide salt can be Ca(OH)₂, Mg(OH)₂, or a combinationthereof. The hydroxide salt can be Ca(OH)₂.

In some aspects, the metal carbonate salt is CaCO₃, the alkaline earthmetal or alkali metal is calcium, the metal halide salt is CaCl₂, thehydrohalic acid is HCl, and the hydroxide salt is Ca(OH)₂.

The hydrolyzing of the metal halide salt can be performed under anysuitable conditions. The hydrolyzing of the metal halide salt can beperformed at any suitable pressure, such as at a pressure of 0.1 MPa−100 MPa, or 0.1 MPa to 9 MPa, or 1 MPa to 9 MPa, or 3 MPa to 9 MPa, or5 MPa to 7 MPa, or less than, equal to, or greater than 0.1 MPa, 0.5, 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 25, 30, 40, 50, 60, 70,80, 90, or 100 MPa. The hydrolyzing of the metal halide salt can beperformed at any suitable temperature, such as a temperature of roomtemperature to 1000° C., or room temperature to 500° C., or 300° C. to500° C., or 350° C. to 450° C., or less than, equal to, or greater thanroom temperature (e.g., about 20° C.), 25, 30, 35, 40, 45, 50, 60, 80,100, 125, 150, 175, 200, 225, 250, 275, 300, 320, 340, 350, 360, 370,380, 390, 400, 410, 420, 430, 440, 450, 460, 480, 500, 550, 600, 650,700, 750, 800, 850, 900, 950, or 1000° C.

The hydrolyzing of the metal halide salt (i.e., in water) produces thehydrohalic acid. The hydrohalic acid can be produced in a phase that isdistinct from the brine solution that includes the water and the metalhalide salt. The acid/water phase can be a vaporous phase, asupercritical water phase, a gaseous phase, or a combination thereof,depending on the hydrolysis conditions used to form the hydrohalic acid.The hydrolyzing of the metal halide salt can produce the hydrohalic acidat any suitable concentration (e.g., in the distinct acid/water phase),such as at a molar content of 0.01% to 10%, or a molar content of 0.1%to 1%, or less than, equal to, or greater than 0.01%, 0.05, 0.1, 0.2,0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.8, 2,2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or 10%molar content.

The reacting of the hydrohalic acid with the metal carbonate salt can beperformed under any suitable conditions. The reacting of the hydrohalicacid with the metal carbonate salt can be performed in the same reactorwith, and using the same conditions as, the hydrolysis of the metalhalide salt to form the hydrohalic acid (e.g., a heated and pressurizedreactor). The reacting of the hydrohalic acid with the metal carbonatesalt can be performed in a separate reactor from the reacting of thehydrolysis of the metal halide salt to form the hydrohalic acid, such asby removing the hydrohalic acid from the reactor, cooling the hydrohalicacid, and performing the reacting of the hydrohalic acid with the metalcarbonate salt under different conditions, such as roomtemperature/pressure conditions. The reacting of the hydrohalic acidwith the metal carbonate salt can be performed at a pressure of 0.1 MPa−100 MPa, or 0.1 MPa to 9 MPa, or 1 MPa to 9 MPa, or 3 MPa to 9 MPa, or5 MPa to 7 MPa, or less than, equal to, or greater than 0.1 MPa, 0.5, 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 25, 30, 40, 50, 60, 70,80, 90, or 100 MPa. The reacting of the hydrohalic acid with the metalcarbonate salt can be performed at a temperature of room temperature to1000° C., or room temperature to 500° C., or 300° C. to 500° C., or 350°C. to 450° C., or less than, equal to, or greater than room temperature(e.g., about 20° C.), 25, 30, 35, 40, 45, 50, 60, 80, 100, 125, 150,175, 200, 225, 250, 275, 300, 320, 340, 350, 360, 370, 380, 390, 400,410, 420, 430, 440, 450, 460, 480, 500, 550, 600, 650, 700, 750, 800,850, 900, 950, or 1000° C.

At least some of the metal halide salt formed from the reacting of thehydrohalic acid with the metal carbonate salt can be recycled as atleast some of the metal halide salt in the hydrolyzing of the metalhalide salt to form the hydrohalic acid and the hydroxide salt. Themetal halide salt formed from the reacting of the hydrohalic acid withthe metal carbonate salt can be any suitable proportion of the metalhalide salt used in the hydrolyzing of the metal halide salt to form thehydrohalic acid and the hydroxide salt. For example, the metal halidesalt formed from the reacting of the hydrohalic acid with the metalcarbonate salt is 0.001 wt % to 100 wt % of the metal halide salt usedin the hydrolyzing of the metal halide salt to form the hydrohalic acidand the hydroxide salt, or 80 wt % to 100 wt %, or less than, equal to,or greater than 0.001 wt %, 0.01, 0.1, 1, 2, 3, 4, 5, 6, 8, 10, 15, 20,25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 91, 92, 93, 94,95, 96, 97, 98, 99, 99.9, 99.99, or 99.999 wt %, or 100 wt %.

Various aspects provide a method of treating CaCO₃. The method caninclude hydrolyzing CaCl₂ to form HCl and Ca(OH)₂. The method caninclude reacting the HCl with the CaCO₃, to form CO₂ and CaCl₂, whereinat least some of the CaCl₂ formed from the reacting of the HCl with theCaCO₃ is recycled as at least some of the CaCl₂ in the hydrolyzing ofthe CaCl₂ to form the HCl and the Ca(OH)₂. The method can furtherinclude reacting a used CO₂-capture sorbent with the Ca(OH)₂, to formthe CaCO₃, wherein at least some of the Ca(OH)₂ formed in the hydrolysisof the CaCl₂ to form the HCl and the Ca(OH)₂ is recycled as at leastsome of the Ca(OH)₂ used in the reacting of the used CO₂-capture sorbentwith the Ca(OH)₂. The method can further include reacting Ca(HCO₃)₂ froma water source with the Ca(OH)₂, to form the CaCO₃, wherein at leastsome of the Ca(OH)₂ formed in the hydrolysis of the CaCl₂ to form theHC1 and the Ca(OH)₂ is recycled as at least some of the Ca(OH)₂ used inthe reacting of the Ca(HCO₃)₂ with the Ca(OH)₂.

The method of treating the metal carbonate salt can be used toregenerate hydrated lime (Ca(OH)₂), such as from the precipitatesproduced during lime softening of water. Lime softening is a commontreatment for municipal and industrial water supplies.

The method of treating the metal carbonate salt can be used to producehydrated lime or dolomitic lime (e.g., a mixture of Ca(OH)₂ andMg(OH)₂). This process is already performed at large scale usingcalcination to produce lime for cement, steelmaking, food processing,and many other industries.

The method of treating the metal carbonate sale can be used to process asource of CaCO₃ to convert it into a Ca(OH)₂ product.

Method of Regenerating a CO₂-Capture Sorbent

The method of treating the metal carbonate salt can be used to removeCO₂ from a used CO₂-capture sorbent (e.g., a CO₂-capture sorbent forair). The method can include reacting a used CO₂-capture sorbent withthe hydroxide salt to provide the metal carbonate salt that is acarbonate salt of the metal in the metal halide salt. The usedCO₂-capture sorbent can be any suitable used CO₂-capture sorbent, suchas formed from contacting CO₂ with any suitable CO₂-capture sorbent. Theused CO₂-capture sorbent can be a used hydroxide-based, ammonia-based,and/or amine-based CO₂-capture sorbent. In some examples, the usedammonia-based and/or amine-based CO₂-capture sorbent can include anammonium carbamate, an ammonium carbonate, an ammonium bicarbonate, or acombination thereof. The used CO₂-capture sorbent can be derived fromsorption of CO₂ by a hydroxide-based, ammonia-based (e.g., aqueousammonia and/or ammonium bicarbonate), and/or amine-based CO₂-capturesorbent (e.g., monoethanolamine, diethanolamine,2-amino-2-methyl-1-propanol, methyl diethanolamine, piperazine). TheCO₂-capture sorbent can be a used hydroxide-based CO₂-capture sorbent,such as Ca(HCO₃)₂ (derived from Ca(OH)₂), Mg(HCO₃)₂ (derived fromMg(OH)₂), K₂CO₃ (derived from KOH), Na₂CO₃ (derived from NaOH), or acombination thereof.

The reacting of the used CO₂-capture sorbent with the hydroxide salt toprovide the metal carbonate salt can be performed under any suitableconditions. The reacting of the used CO₂-capture sorbent with thehydroxide salt to provide the metal carbonate salt can be performed at apressure of 0.01 MPa to 10 MPa, 0.05 MPa to 0.2 MPa, or less than, equalto, or greater than 0.01 MPa, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11,0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.25, 0.3, 0.4,0.5, 0.6, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, or 10 MPa.The reacting of the used CO₂-capture sorbent with the hydroxide salt toprovide the metal carbonate salt is performed at a temperature of roomtemperature to 350° C., or 50° C. to 150° C., or 90° C. to 110° C., orless than, equal to, or greater than room temperature 25, 30, 35, 40,45, 50, 60, 70, 80, 85, 90, 95, 100, 105, 110, 115, 120, 120, 125, 150,175, 200, 225, 250, 275, 300, 320, 340, or 350° C.

The method can include contacting a CO₂-capture sorbent with CO₂ to formthe used CO₂-capture sorbent. In other aspects, the CO₂-capture sorbentis contacted with CO₂ to form the used CO₂-capture sorbent prior to theonset of the method. The method can include Ca(OH)₂, Mg(OH)₂, KOH,and/or NaOH with CO₂ to form the used CO₂-capture sorbent.

The reacting of the used CO₂-capture sorbent with the hydroxide salt toform the metal carbonate salt can also form an unused CO₂-capturesorbent, e.g., to regenerate the used CO₂-capture sorbent. The unusedCO₂-capture sorbent can be Ca(OH)₂, Mg(OH)₂, KOH, and/or NaOH. Theunused CO₂-capture sorbent can be KOH and/or NaOH. The method canfurther include providing the unused CO₂-capture sorbent for CO₂capture. The method can further include contacting the regeneratedCO₂-capture sorbent with CO₂ to form a used CO₂-capture sorbent, whichcan then again be regenerated using the method treating the metalcarbonate salt.

In some aspects, none of the hydroxide salt formed in the hydrolysis ofthe metal halide salt to form the hydrohalic acid and the hydroxide saltis recycled as the hydroxide salt used in the reacting of the usedCO₂-capture sorbent with the hydroxide salt. In other aspects, at leastsome of the hydroxide salt of the metal formed in the hydrolysis of themetal halide salt to form the hydrohalic acid and the hydroxide salt canbe recycled as at least some of the hydroxide salt used in the reactingof the used CO₂-capture sorbent with the hydroxide salt. For example,the hydroxide salt of the metal formed in the hydrolysis of the metalhalide salt can be 0.001 wt % to 100 wt % of the hydroxide salt used inthe reacting of the used CO₂-capture sorbent with the hydroxide salt, or80 wt % to 100 wt %, or less than, equal to, or greater than 0.001 wt %,0.01, 0.1, 1, 2, 3, 4, 5, 6, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55,60, 65, 70, 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.9,99.99, or 99.999 wt %, or 100 wt %.

The method of regenerating a used CO₂-capture sorbent can includehydrolyzing a metal halide salt to form a hydrohalic acid and ahydroxide salt of the metal in the metal halide salt, the metalincluding an alkaline earth metal or an alkali metal. The method caninclude reacting the used CO₂-capture sorbent with the hydroxide salt,to form a carbonate salt of the metal in the metal halide salt. Themethod can also include reacting the hydrohalic acid with the carbonatesalt, to form CO₂ and the metal halide salt, wherein at least some ofthe metal halide salt formed from the reacting of the hydrohalic acidwith the carbonate salt is recycled as at least some of the metal halidesalt in the hydrolyzing of the metal halide salt to form the hydrohalicacid and the hydroxide salt.

The method of regenerating a used CO₂-capture sorbent can includehydrolyzing CaCl₂ to form HC1 and Ca(OH)₂. The method can includereacting the used CO₂-capture sorbent with the Ca(OH)₂, to form CaCO₃.The method can also include reacting the HCl with the CaCO₃, to form CO₂and CaCl₂, wherein at least some of the CaCl₂ formed from the reactingof the HCl with the CaCO₃ is recycled as at least some of the CaCl₂ inthe hydrolyzing of the CaCl₂ to form the HCl and the Ca(OH)₂.

The method of regenerating a used CO₂-capture sorbent can provide alower-temperature and lower-energy alternative to high-temperaturecalcination for hydroxide-based CO₂ DAC solvents. Solvents like KOH andNaOH have high affinity for CO₂, even at low partial pressure, and offerdesign advantages for the massive air contactors required forlarge-scale CO₂ removal from the atmosphere. For instance, being liquid,these capture solvents can be circulated over a variety of air contactorgeometries while being regenerated at a central location, thereby usingthe full system capacity for continuous CO₂ capture. However,regeneration of these solvents requires decomposition of a carbonate(commonly CaCO₃) which is an energy-intensive, high-temperature (900°C.) process using existing calcination techniques. This energyrequirement is a significant drawback for hydroxide solvents since itresults in additional emissions that must be offset by DAC systemcapacity to produce a net CO₂ reduction. Instead of high-temperaturecalcination, the present method of regenerating a CO₂-capture sorbentcan decompose carbonates using a regenerable acid produced from thehydrolysis of a chloride-based regeneration solution. A comparison ofthe process using CaCl₂ brine to calcination regeneration is shown inFIG. 1 .

The process of brine hydrolysis regeneration in FIG. 1 can integratewith DAC systems by using hydrochloric acid (HCl) to decomposecarbonates, thus releasing captured CO₂ and recovering the precipitatedhydroxide (e.g., Ca[OH]₂) to regenerate soluble hydroxide solvents suchas KOH and NaOH. Hydrolysis can occur at significantly lowertemperatures than calcination (e.g., 400° versus 900° C.), and it canoffer a feasible way to recycle thermal energy released from Ca(OH)₂formation, thus lowering the quantity of input thermal energy. Chloridecompounds in the regeneration solution are not consumed and can becontinually recycled.

Brine hydrolysis can be the source of HCl used for carbonatedecomposition, and it has been observed experimentally. A plot ofpressure versus concentration for CaCl₂ brine is shown in FIG. 2 , whichis a plot of phase composition data for CaCl₂ brine near its criticalpoint (see, Bischoff, J.; Rosenbauer, R.; Fournier, R. The Generation ofHCl in the System CaCl₂-H₂O: Vapor—Liquid Relations from 380°-500° C.Geochimica et Cosmochimica Acta 1996, 60 (1), 7-16). As the data show,HCl is produced in significant amounts during the dynamic equilibriumabove CaCl₂ brine held at a moderate temperature of 400° C. Thehydrolysis data shown in FIG. 2 also result in corresponding Ca(OH)₂left behind in the brine, some of which precipitates because of itsdecreasing solubility with temperature. The hydrolysis data highlightedin FIG. 2 evaluated conditions over the temperature range of 380° to500° C. but was a study of equilibrium conditions and, by definition,did not consider the kinetics of CaCl₂ hydrolysis.

Referring to the brine hydrolysis regeneration process in FIG. 1 , thehydrolysis reactor contains a concentrated solution of CaCl₂ and H₂Owith an acid/water phase above it. The acid/water phase can include H₂Owith a fraction of HCl produced as a result of brine hydrolysis; theexact amount of HCl depends on the temperature, pressure, and brinecomposition within the reactor. The other component from CaCl₂hydrolysis, Ca(OH)₂, can remain in the brine and can precipitate becauseof its decreasing solubility with temperature. At equilibrium, theamount of CaCl₂ hydrolysis can be stable, but in the brine hydrolysisregeneration scheme, HCl-containing vapor can be reacted with CaCO₃ fromcausticization of a DAC solvent. This HCl consumption along with Ca(OH)₂precipitation can shift the reaction toward continued hydrolysis. Thedecomposition of CaCO₃ can release a stream of captured CO₂ and canreproduce the CaCl₂ salt to complete the cycle.

Some indication of the minimum theoretical energy requirements can begained by considering their standard heat of reaction. Table 1 comparesthe reaction energies for brine hydrolysis regeneration and the existingmethod using calcination and lime slaking; these pathways correspond tothe alternatives shown in FIG. 1 . Summing the reaction energies forboth pathways gives the same net endothermic energy of +113 kJ/mol CO₂,which is a thermodynamic necessity since all inputs and outputs at theprocess boundary are assumed to be identical. In practice, however,recovering energy between process steps is not always feasible. Forinstance, lime slaking is significantly exothermic, but the reactiondoes not proceed in the forward direction at the temperature needed forcalcination (900° C.), and as a result, heat from this reaction cannotbe used to offset the calcination energy requirement of +178 kJ/mol CO₂(without the input of additional work).

TABLE 1 Comparison of Regeneration Pathway Energies. ExistingCalcination Regeneration Brine Hydrolysis Regeneration Reaction Heat ofReaction Reaction Heat of Reaction Carbonate Calcination +178 kJ/mol CO₂Carbonate Decomposition  −15 kJ/mol CO₂ CaCO₃ → CaO + CO₂ CaCO₃ + 2HCl →CaCl₂ + CO₂ + H₂O Lime Slaking  −65 kJ/mol CO₂ Brine Hydrolysis +128kJ/mol CO₂ CaO + H₂O → Ca(OH)₂ CaCl₂ + 2H₂O → 2HCl + Ca(OH)₂

In contrast to calcination regeneration, the reactions including brinehydrolysis regeneration in Table 1 can both proceed under the sameconditions of temperature and pressure, and as a result, it istheoretically possible to reduce the regeneration energy requirementfrom +178 kJ/mol CO₂ to +113 kJ/mol CO₂. Even if the steps of carbonatedecomposition and brine hydrolysis are not combined in the same reactor,the full energy required by brine hydrolysis alone is still asignificant savings compared to calcination (+128 kJ/mol CO₂ versus +178kJ/mol CO₂) and occurs at a much lower temperature (400° C. versus 900°C.).

FIG. 3 presents a KOH solvent capture process using Ca causticization, aDAC process suitable for large-scale application. In the system, a KOHsolvent contacts air and absorbs CO₂. Spent CO₂-rich solvent is thencausticized using Ca(OH)₂ where the captured CO₂ is transferred from thesolvent to an insoluble carbonate, CaCO₃ in this case. Causticization ismildly exothermic, but its primary utility is to transfer CO₂ from theliquid solution to a solid, thereby reducing sensible energy consumptionduring regeneration. At this point in the process, precipitated CaCO₃can enter the brine hydrolysis regeneration stage where it is decomposedusing HCl to release the captured CO₂. The resulting CaCl₂ salt reformsthe brine used for HCl and Ca(OH)₂ generation.

Nominal conditions within the brine hydrolysis process have beenestimated at 400° C. and 6 MPa based on the experimental data shown inFIG. 2 .

While hydroxide-based solvents such as KOH and NaOH have desirablecapture and engineering properties, their high regeneration energyrequirements (greater than 178 kJ/mol CO₂ at 900° C.) necessitates thedevelopment of improved alternatives. The method of the presentinvention provides a practical means to approach the theoretical limitof regeneration energy (113 kJ/mol CO₂) at a significantly lowertemperature (400° C.), thereby providing improved CO₂ separationperformance over the options available today.

Method of Removing CO₂ from Water

The method of treating the metal carbonate salt can be used to removeCO₂ from water. For example, the method can further include reacting abicarbonate salt such as NaHCO₃, Mg(HCO₃)₂, Ca(HCO₃)₂, KHCO₃, or acombination thereof, taken from any suitable water source, with thehydroxide salt to provide the metal carbonate salt that is a carbonatesalt of the metal in the metal halide salt. The method can be a methodof softening water. The water source can be a natural water source, suchas salt water, ocean water, brackish water, fresh water, a stream, apond, a lake, a river, or a combination thereof. The bicarbonate saltcan be Ca(HCO₃)₂.

In some aspects, none of the hydroxide salt of the metal formed in thehydrolysis of the metal halide salt to form the hydrohalic acid and thehydroxide salt is recycled as at least some of the hydroxide salt usedin the reacting of the bicarbonate salt with the hydroxide salt. Inother aspects, at least some of the hydroxide salt of the metal formedin the hydrolysis of the metal halide salt to form the hydrohalic acidand the hydroxide salt is recycled as at least some of the hydroxidesalt used in the reacting of the bicarbonate salt with the hydroxidesalt. For example, the hydroxide salt of the metal formed in thehydrolysis of the metal halide salt is 0.001 wt % to 100 wt % of thehydroxide salt used in the reacting of the bicarbonate salt with thehydroxide salt, or 80 wt % to 100 wt %, or less than, equal to, orgreater than 0.001 wt %, 0.01, 0.1, 1, 2, 3, 4, 5, 6, 8, 10, 15, 20, 25,30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 91, 92, 93, 94, 95,96, 97, 98, 99, 99.9, 99.99, or 99.999 wt %, or 100 wt %.

The method of removing CO₂ from water can include hydrolyzing a metalhalide salt to form a hydrohalic acid and a hydroxide salt of the metalin the metal halide salt, the metal including an alkaline earth metal oran alkali metal. The method can include reacting a bicarbonate salt froma water source with the hydroxide salt, to form a carbonate salt of themetal in the metal halide salt. The method can also include reacting thehydrohalic acid with the carbonate salt, to form CO₂ and the metalhalide salt, wherein at least some of the metal halide salt formed fromthe reacting of the hydrohalic acid with the carbonate salt is recycledas at least some of the metal halide salt in the hydrolyzing of themetal halide salt to form the hydrohalic acid and the hydroxide salt.

The method of removing CO₂ from water can include hydrolyzing CaCl₂ toform HCl and Ca(OH)₂. The method can include reacting Ca(HCO₃)₂ from awater source with the Ca(OH)₂, to form CaCO₃. The method can includereacting the HCl with the CaCO₃, to form CO₂ and CaCl₂, wherein at leastsome of the CaCl₂ formed from the reacting of the HCl with the CaCO₃ isrecycled as at least some of the CaCl₂ in the hydrolyzing of the CaCl₂to form the HCl and the Ca(OH)₂.

An example of the method of removing CO₂ from water is shown in FIG. 4 ,which illustrates a method for removing CO₂ from the ocean.Advantageously, the method leverages two advantages of CaCl₂ splittingcompared to a NaCl-based process, resulting in a transformationalimprovement in ocean CO₂ removal. Firstly, the Ca and Cl constituents ofthe CaCl₂ brine can be recycled, and when regenerated, the brine isalready concentrated. This is unlike a NaCl-based process where thesplit NaOH and HCl constituents are released with the treated seawaterand replacement NaCl brine must be reconcentrated. Secondly, theproposed method of CaCl₂ splitting is based on the hydrolysis of CaCl₂brine, a thermochemical process where appreciable quantities of HCl areproduced in the acid/water phase above a heated CaCl₂ brine pool.Compared to electrochemical- and/or membrane-based salt-splittingapproaches, brine hydrolysis is potentially more robust and lower costto operate. In various aspects, the method can lower the cost of oceanCO₂ removal to an estimated cost of $62/ton CO₂ or less.

Aspects of the method address the high energy costs needed to driveocean CO₂ removal. Input energy is required to convert the dominant formof CO₂ in the oceans, bicarbonate ion (HCO₃ ⁻), to form CO₂ gas that canbe separated for utilization or sequestration. Conceptually, thisprocess can be shown as HCO₃ ⁻(aq) CO₂ (g)+OH⁻ (aq), which has areaction heat of +66 kJ/mol CO₂. Unfortunately, real processes have notachieved this minimum level of energy consumption, and state-of-the-artapproaches are estimated to require many times this amount of energy tocomplete the task of CO₂ removal. Excessive energy consumption hindersocean CO₂ removal by increasing operating costs and creating additionalCO₂ emissions that need to be offset. The method of the presentinvention of removing CO₂ from water can provide a thermochemical cycleto achieve production of CO₂ from bicarbonate while enabling a lowercost of energy consumption compared to what is possible today, makingthe concept of ocean CO₂ removal a more feasible tool for carbonmanagement.

Various aspects of the method of removing CO₂ from water is a process ofhydrolytic softening based on salt splitting, but instead of NaCl thetargeted salt can be CaCl₂, as shown in FIG. 4 . Using CaCl₂ can allowfor a thermochemical approach to separate the salt into acid and base,thus switching the primary form of energy input from electricity toheat. The use of CaCl₂ can also avoid the need to concentrate the brineprior to splitting which is an energy-intensive requirement for someNaCl-based approaches.

Hydrated lime, Ca(OH)₂, is used in Step 1 of FIG. 4 to removebicarbonate ions in seawater using the familiar chemistry of limesoftening: at elevated pH, bicarbonate ions are reduced to carbonatewhich forms solid precipitates of CaCO₃. The alkalinity of seawater isapproximately 200 ppm (as CaCO₃), and cold lime softening can beexpected to achieve a reduction to around 50 ppm with nearstoichiometric utilization of Ca(OH)₂. The precipitates formed in Step 1can be allowed to settle by gravity and are collected as a dense slurry.Softened seawater exits the process; its elevated pH to be dissipated byabsorbing additional CO₂ from the atmosphere and mixing with untreatedseawater. These mixing processes can be accelerated by mechanical means,such as to prevent harm to the local environment.

The CaCO₃ slurry produced from the seawater softening process in FIG. 4can be collected and sent to a reactor where it is mixed with aqueousHCl from brine hydrolysis. The resulting spontaneous reaction (−15kJ/mol CO₂) decomposes the CaCO₃ to liberate CO₂ gas and reform theCaCl₂ brine. In order to control the concentration of the resultingCaCl₂ brine, the density of the incoming CaCO₃ slurry can be controlledalong with the concentration of the HCl solution leaving the brinehydrolysis step. Advantageously, as compared to an a NaCl-based process,the need for brine concentration before salt splitting is avoided. Witha NaCl process, the split constituents of HCl and NaOH are lost to thetreated seawater, and new concentrate must be continually reformed.

The CO₂ released during Step 2 can include associated water vapor butcan otherwise be of high purity. The CaCO₃ slurry can act as aneffective scrubbing solution for vapor-phase HCl to prevent itscontamination of the CO₂ product. The reaction between aqueous HCl andslurry CaCO₃ can be conducted under pressure to produce a pressurizedCO₂ product, thus saving gas compression energy input.

Brine hydrolysis can be a driving process behind hydrolytic softening,as shown in FIG. 4 . In Step 3, the concentrated CaCl₂ brine, at avolume flow roughly 0.025% that of the seawater, can be hydrolyzed toform Ca(OH)₂ for softening and HCl to decompose the CaCO₃ precipitate.

As the data in FIG. 2 show, HCl is produced in significant amounts(approximately 3000 ppm) during the dynamic equilibrium above CaCl₂brine held at a moderate temperature of 400° C. The hydrolysis datashown in FIG. 2 also result in corresponding Ca(OH)₂ left behind in thebrine, some of which can precipitate because of its decreasingsolubility with temperature.

In addition to the hypothesized energy savings with brine hydrolysis,other potential advantages have been identified over electrochemicalapproaches to salt splitting. For instance, the process chemistry isrobust and can be tolerant of the other dissolved species found inseawater. Any impurities that accumulate in the process can be kept incheck with a periodic blowdown of the excess seawater Ca thatprecipitates in addition to the Ca added from the hydrated lime.Finally, unlike electrochemical processes, there are no concerns ofcatalyst or membrane fouling with hydrolytic softening.

Compared to a NaCl-based process, the hydrolytic softening methoddescribed herein can be less disruptive to ocean life since it does notacidify the water which could harm sensitive organisms. As a result,hydrolytic softening can present a relatively lower environmental riskand should face fewer restrictions on its application. Regardingoffshore processing costs, the simple reactor needs of hydrolyticsoftening make it more likely that cost projections can be met comparedto a more complex process based on EDBM that requires large membranesurfaces that must be kept clean for optimal efficiency. In contrast, afloating reactor for hydrolytic softening only needs to separateseawater undergoing treatment from its surroundings and provide acollection basin (such as the ocean floor) for the precipitatedcarbonates. As with environmental concerns, hydrolytic softening appearsto present less technical risk regarding ocean process developmentcompared to state-of-the-art alternatives, and this feature shouldtranslate into a shorter time to market.

The method of hydrolytic water softening can be used for variouspurposes including ocean CO₂ removal and treatment of industrial wastebrines. The application of treating waste brines to make them easier torecycle or to recover valuable products therefrom can include treatingproduced water from oil and gas development, or displaced brine fromgeologic CO₂ sequestration. Such processes could be powered by naturalgas in remote areas.

EXAMPLES

Various aspects of the present invention can be better understood byreference to the following Examples which are offered by way ofillustration. The present invention is not limited to the Examples givenherein.

The reactor used in Examples 1 and 3 can decompose Ca and Mg carbonates(i.e., CaCO₃ and MgCO₃) as an alternative to conventional calcination(i.e., the direct thermal decomposition of these compounds. The reactoruses aqueous solutions of the corresponding chloride salts (i.e., CaCl₂and MgCl₂) as a reaction intermediary to achieve carbonate decompositionat lower temperatures than those needed for direct thermaldecomposition. Furthermore, the reactor converts the resulting oxide(CaO and MgO) into the corresponding hydroxide (Ca(OH)₂ or Mg(OH)₂),which is an exothermic reaction. This release of energy has thepotential to offset the energy required for carbonate decomposition, atask that is virtually impossible to achieve with conventionalcalcination.

The reactor is illustrated in FIG. 5 , which shows the cross section ofthe reactor for the decomposition of CaCO₃ from CO₂-capture processes,water softening processes, and/or natural limestone. Within the sealedreactor a pool of concentrated liquid CaCl₂ brine resides at the bottomwith a vapor phase above it. The vapor phase will consist of H₂O with asmall fraction of HCl gas produced as a result of CaCl₂ salt hydrolysis;the exact amount of HCl depends on the temperature, pressure and brineconcentration within the reactor. The other component of CaCl₂hydrolysis, CaO, remains in the brine and is converted to the hydroxideCa(OH)₂, which can precipitate due to its low solubility. At equilibriumthe degree of CaCl₂ hydrolysis is fixed, but in the reactor shown, solidCaCO₃ is suspended in the vapor phase which promotes continualhydrolysis of the salt by consuming HCl gas to reproduce CaCl₂ salt. CO₂gas is produced during this process which can be released from thereactor along with some amount of H₂O vapor. Provided the CaCO₃ beddepth is maintained, HCl vapor can be scrubbed from the gases exitingthe reactor.

Conventional calcining of CaCO₃ can be represented by Reaction 1 whichrequires a 900° C. or higher temperature. In comparison, the proposedprocess using hydrolyzed CaCl₂ can be imagined to consist of the threesteps shown in Reactions 2-4, where Reaction 2 is salt hydrolysis,Reaction 3 is the acidic decomposition of the carbonate, and Reaction 4is hydroxide formation. Reactions 2 and 3 equate to the thermaldecomposition of Reaction 1 and sum to the same theoretical heat ofreaction, 178 kJ/mol CO₂. However, by incorporating exothermic hydroxideformation, Reaction 4, the overall alternative process has a reducedtheoretical energy requirement of 112 kJ/mol CO₂. In addition to this37% reduction in theoretical energy use, the alternative process has thepotential to lower the required heat source temperature since Reaction 2has been shown in the literature to occur at more moderate temperaturesof 400° C. -500° C.CaCO₃→CaO+CO₂(+178 kJ/mol CO₂)  (Reaction 1)CaCl₂+H₂)→CaO+2HCl(+113 kJ/mol CO₂)  (Reaction 2)(+65 kJ/mol CO₂)  (Reaction 3)CaO+H₂O→Ca(OH)₂(−66 kJ/mol CO₂)  (Reaction 4)

Reactor pressure is another parameter for the process. The pressure canbe selected in accordance with the temperature to maintain a distinctbrine phase and another phase that includes water and the produced acid(e.g., supercritical water, such as above the brine stage), since theseare required for the desired separations to take place. At thetemperatures under consideration, the low pressure extreme results insolidified chloride salt in an atmosphere of H₂O and HCl vapors. At theother extreme where pressure is too high, a dense phase liquid is formedwith no distinct separation of hydrolysis products or CO₂. In this caseCO₂ separation would be infeasible and the solid carbonate reactant(e.g. CaCO₃) and hydroxide product (e.g. Ca(OH)₂) would be mixed.Precipitation of the hydroxide on the carbonate may also block completeconversion and decrease the efficiency of sorbent recycling.

The reactor operating pressure can also be used to advantage for theproduction of pressurized CO₂ that does not require any, or as much,compression for pipeline transport or geologic sequestration. The otherstreams entering and exiting the reactor are solids (e.g., the carbonateand hydroxide). Conveying these materials through a pressure gradientmay pose engineering challenges but since the materials areincompressible, these streams should not require excessive compressionenergy.

Example 1. Regeneration Solution for CO₂ Direct Air Capture Solvents

A laboratory-scale apparatus is used to generate the data necessary toidentify the preferred regeneration solution composition from a selectedset of brine chemistries, identify effective hydrolysis conditions, andprovide a basis for modeling the mass and energy flows with anintegrated DAC process. Capture of CO₂ from the atmosphere usinghydroxide-based solvents like KOH and causticizing them to form CaCO₃has been demonstrated elsewhere and it is not necessary to include thesesteps as part of this evaluation. Key processes include hydrolysis ofthe chloride-based brine to form HCl and precipitated Ca(OH)₂,decomposition of CaCO₃ under hydrolysis temperature and pressureconditions, and recovery of CO₂ gas.

The laboratory apparatus is diagrammed in FIG. 6 . The apparatusincludes a heated tube reactor with provisions for makeup liquidinjection and an off-gas conditioning and measurement train. The reactorserves as a vessel for brine hydrolysis and as the reaction chamber forcarbonate decomposition by fitting a solids basket above the liquidbrine. The tubular reactor is fitted with a reacting solids basket,temperature and pressure transducers, and a vapor extraction probe. Anoff-gas analysis train is assembled to depressurize the vapors, allowfor composition analysis, and totalize gas production.

Individual test series can have slight variations in setup and operatingprotocols depending on their specific objective, but all tests followthe same overall process. Prior to testing, the reactor is loaded with apredetermined quantity and composition of CaCl₂ as brine or solid, andfor selected cases, CaCO₃ (without and with impurities). After heatingto the test condition, vapor is extracted and its composition and flowrate recorded as a function of time. Condensed vapor samples forsubsequent analysis can also be collected. The reactor operates in anopen circuit mode; i.e., water vapor that would normally be recycledwithin the reactor is vented, and as a result, extended tests canrequire the addition of makeup water using a high-pressure pump.Following cooldown, the reactor is opened, and residual liquid and solidsamples are collected for composition and other needed analysis.

Testing includes several evaluation stages, eventually leading tosemicontinuous tests of the regeneration method. Initial tests toevaluate brine hydrolysis and carbonate decomposition are batch-operatedwithout the addition of material inputs. Parameters includingtemperature over a range of 300° to 500° C. and brine composition areevaluated for their effect on hydrolysis and HCl formation. Hydrolysisis evaluated by analysis of the off-gas for HCl and the post-testanalysis of recovered reactor liquids and precipitated solids. Operatingpressure is constrained by temperature and brine composition and isdetermined for each test. Brine composition has been shown to have aneffect on hydrolysis when comparing synthetic versus natural seawater,and is evaluated here in more depth by testing up to five brinechemistries to identify the preferred solution composition. Allsolutions are chloride-based, and one is aqueous CaCl₂. Following thehydrolysis evaluation, carbonate (i.e., CaCO₃) decomposition isevaluated at conditions suitable for brine hydrolysis to determine ifthis process can be incorporated within the hydrolysis reactor forsimplified reactant transport and potential energy integration. Off-gasanalysis is used to estimate the rate of conversion, and post-testsample recovery is used to determine conversion extent.

Testing then advances to longer-duration semicontinuous runs to evaluatemechanisms necessary for successful cycle operation. Key mechanismsinclude the cycling of HCl and CaCl₂, the precipitation and separationof Ca(OH)₂, the decomposition of CaCO₃ in a physical form it is likelyto be in after causticization (i.e., precipitated from solution), theextraction of CO₂ product, and the ability to pass impurity species fromother parts of the DAC process (e.g., KOH/K₂CO₃). This issemicontinuous, where CaCO₃ solids are charged at the beginning of a runand product Ca(OH)₂ are recovered after, but CO₂ product vapors arewithdrawn continuously, and makeup brine water is added as needed tosustain operation. Before testing, the reactor is charged with a largerquantity of the target carbonate to allow for longer, semicontinuousevaluation of regeneration cycle processes, including the precipitationand separation of Ca(OH)₂, the decomposition of CaCO₃ in a physical formit is likely to be in after causticization, and extraction of CO₂ andany associated vapors. Testing can identify a baseline condition thatcan serve as the basis for high-level process integration modeling.

Data collection includes a combination of operational data logging andposttest analysis of recovered samples. Data logging includes reactortemperature(s) and pressure and analysis of the off-gas composition andflow rate. Recovered samples from each test include residual brineliquid, precipitated solids in the brine, and residual solids left inthe carbonate loading basket. Liquids undergo analysis for pH anddissolved species determination. Solids are evaluated for their chemicalmakeup using X-ray fluorescence and, as needed, X-ray diffraction formineral phase identification and inspection using a scanning electronmicroscope.

The regeneration process is evaluated over a range of temperatures from300° C. to 500° C. Below this range, hydrolysis diminishes because ofreduced HCl vapor pressure, and above it, Ca(OH)₂ formation is notfavored. Operating pressure is constrained by temperature and brinecomposition and will be determined individually for each test. Usingavailable data, a typical operating pressure is expected to be 6 MPa.

Input solids include a target carbonate compound, CaCO₃, which isrepresentative of the final capture product for DAC systems utilizing Cacausticization. Initial batch conversion tests use a purchased CaCO₃reagent for test-to-test consistency, but the semicontinuous tests useCaCO₃ precipitated from a simulated causticization process. Thisprecipitated material includes or is spiked with process impurities suchas unconverted Ca(OH)₂ and carryover KOH/K₂CO₃ to determine their fateand demonstrate that a manageable steady state can be achieved. Ambientpollutant impurities, specifically SO₂ and NOR, are not be evaluatedexperimentally, but they will be treated using modeling to identifytheir likely fate and explore management options.

Chemical process modeling is used to supplement the results andextrapolate performance for a full-scale DAC. In order to estimate thepotential performance of a full-scale DAC system, process modelingsoftware Aspen Plus is used alongside experimental data to produce acomplete analysis.

The proposed effort is directly relevant to the development of improvedDAC systems by addressing a key barrier to commercialization for solventDAC, i.e., the regeneration energy it requires. Cuts in regenerationenergy compared to high-temperature calcination result in feweremissions that must be offset to achieve net carbon reduction, andlowering the maximum heat source temperature expands the pool ofcandidate energy sources that can be applied to power large-scale DAC.Even if this approach may not result in the lowest specific separationenergy or the lowest regeneration temperature compared to sorbent- ormembrane-based approaches, it will still be impactful because of theengineering advantages solvents offer to the design of large-scale aircontactors, in particular the ability to decouple CO₂ capture fromregeneration. Therefore, feasible methods to reduce the energyconsumption of DAC solvents can be used to implement large-scale aircontactors based on solvents in the near term, but the same may not betrue for sorbent- or membrane-based systems.

Example 1 Supporting Data

The process of hydrolyzing CaCkl₂ to form HCl and Ca(OH)₂ wasinvestigated experimentally to demonstrate the potential energy savingsof brine hydrolysis regeneration over the conventional approach based onhigh-temperature calcination. The apparatus diagrammed in FIG. 6 wasused to determine the degree of CaCl₂ hydrolysis as a function oftemperature, and to experimentally determine the standard heat ofreaction, which has a theoretical value of +128 kJ/mol CO₂ as shown inTable 1 for the brine hydrolysis reaction itself.

For the experiments, approximately 20 g of hydrated CaCl₂ was loadedinto the vertical tube reactor of FIG. 6 and heated to temperatures overthe approximate range of 250° C. to 490° C. Reactor pressure wasmaintained at approximately 0.1 MPa absolute, and the salt was exposedto a steam atmosphere generated from the vaporization of makeup waterpumped into the heated cabinet. Steam flow was maintained by the watermakeup pump and equated to a volume flow rate of approximately 3.7 Lpmat the exit conditions of the heated cabinet (i.e., 0.1 MPa and 191°C.). Hydrolysis extent was monitored by condensing the steam atmosphereexiting the reactor cabinet, and measuring condensate pH, which wascorrelated to HCl concentration. Periodic samples of this condensatewere also collected and analyzed for calcium and chloride ions toprovide confirmation of its composition.

Summary data for the CaCl₂ hydrolysis experiments are presented in Table2 and in the Arrhenius plot of FIG. 7 . Within experimental variability,the data clearly show a linear trend with a curve-fit slope of −7683degrees Kelvin. Given that the slope of an Arrhenius plot is equal tothe negative value of activation energy divided by the universal gasconstant (8.31451 J/mol/K), the experimentally-determined activationenergy was +127.8 kJ/mol Ca (or per mole CO₂ if discussing the entireregeneration process). This value is virtually identical to thetheoretical value of CaCl₂ brine hydrolysis presented in Table 1, and itprovides evidence of the reduced activation energy requirement of thismethod compared to calcination regeneration.

TABLE 2 Summary Data from CaCl₂ Hydrolysis Experiments. Reactor ReactorMeasured HCl Pressure, Temperature, Content in Gas MPa Absolute ° C.Phase, ppmv 0.1 253 43 0.1 290 104 0.1 350 310 0.1 351 331 0.1 382 4640.1 435 1140 0.1 487 5160

Example 2. Energy Estimation for DAC Regeneration Solution

The state point data table applicable to various solvent materials isprovided in Table 3. The entries for the pure solvent, working solution,and absorption fields in Table 3 are based on a DAC system using KOHcapture solvent and Ca causticization as shown in FIG. 3 . Improvementsfrom using the regeneration solution and brine hydrolysis regenerationappear under the desorption field; these values are estimated based onthe minimum-pressure CaCl₂ hydrolysis condition presented in FIG. 2 .However, it is important to note that those data were not gathered tooptimize HCl production and that a wider range of temperature conditions(300° to 500° C.) and multiple brine compositions are evaluated inExample 1 to optimize DAC system integration.

TABLE 3 Preliminary state point data table. Measured/ Estimated UnitsPerformance Pure Solvent^(a) Molecular Weight mol⁻¹ 56.1 Normal BoilingPoint C 1327° Normal Freezing Point C  405° Vapor Pressure at 15° C. bar0 Working Solution^(b) Concentration kg/kg 0.10 Specific Gravity — 1.09(15° C./15° C.) Specific Heat Capacity at kJ/kg · K 3.88 STP Viscosityat STP cP 1.25 Surface Tension at STP dyn/cm 76.5 CO₂ Mass TransferRate, m/s 0.0013 [K_(L)] CO₂ Reaction Rate — 75% over 5 s ThermalConductivity W/(m · K) 0.616 Absorption^(b) Pressure bar 1 Temperature C15 Equilibrium CO₂ Loading gmol CO₂/kg 0.46 Heat of Absorption kJ/kg CO₂2180 Solution Viscosity cP 1.25 Desorption Pressure bar <50^(c)Temperature C 300 to 350^(c) Equilibrium CO₂ Loading gmol CO₂/kg0.002^(d) Heat of Desorption kJ/kg CO₂ 3800 to 5700^(e) ^(a)Measuredproperty data based on KOH as the pure solvent; ^(b)Reported data basedon CO₂ DAC with a 2M KOH solution; ^(c)Projected values from extensionof reported data; ^(d)Projected loading assuming an optimized CaCO₃conversion of 99.5%; ^(e)Projected range based on similar low and highheat recuperation assumptions used for solvent DAC analysis.

The heat of desorption value in Table 3 represents an approximately 35%energy savings compared to conventional CaCO₃ calcination. For furthercomparison, recent analyses for solid DAC sorbents gave a range of 3400to 4800 kJ/kg CO₂ for regeneration energy. The correspondingregeneration temperature for that sorbent analysis was assumed to be 67°to 100° C., but many engineering issues need to be overcome to realizethe large-scale application of potential. The data presented in Table 3demonstrates that brine hydrolysis regeneration can combine thedesirable CO₂ capture and system engineering characteristics of solventDAC with the lower energy input requirements more typical of solidsorbent processes.

Example 3. Hydrolytic Softening of Ocean Water for Carbon DioxideRemoval

Laboratory testing is used to identify effective hydrolysis conditionsand provide a basis for modeling the mass and energy flows for anintegrated hydrolytic softening process, as illustrated in FIG. 4 .Parametric brine hydrolysis testing is performed where data will begenerated regarding the extent of hydrolysis conversion at variousconditions of temperature, brine composition, and vapor extraction rate.Hydrolytic lime product testing is performed, and favorable hydrolysisconditions identified during the parametric tests are repeated forextended durations to produce larger quantities of the precipitatedsolids (referred to as hydrolytic lime) for subsequent softeningeffectiveness testing.

The laboratory apparatus is diagrammed in FIG. 6 ; it is based around ahigh-temperature (1000° C. maximum) vertical tube reactor system. Theapparatus includes a heated tube reactor with provisions for makeupliquid injection and an off-gas conditioning and measurement train.

For each semi-batch evaluation test, the reactor is loaded with apredetermined quantity and composition of brine. After heating to thetest condition, vapor is extracted and its composition and flow raterecorded as a function of time. Composition data is determined using anonline Fourier transform infrared gas analyzer that includes acalibration for HCl. The gas is also passed through an absorbingimpinger solution to capture the acid gas and allow determination of atotal acid quantity. For these tests, the reactor operates in an opencircuit mode; i.e., water vapor that would normally be recycled withinthe reactor will be vented, and as a result, extended tests may requirethe addition of makeup water using a high-pressure pump. Followingcooldown, the reactor is opened, and residual liquid and solid samplesare collected for yield determination and composition analysis.

Data collection includes a combination of operational data logging andposttest analysis of recovered samples. Data logging includes reactortemperature(s) and pressure and analysis of the off-gas composition andflow rate. Recovered samples from each test include residual brineliquid and precipitated solids in the brine. Liquids undergo analysisfor pH and dissolved species determination. Solids are evaluated fortheir chemical makeup using X-ray fluorescence and, as needed, X-raydiffraction for mineral phase identification and inspection using ascanning electron microscope.

Parametric Brine Hydrolysis Testing. Parameters including temperatureand brine composition are evaluated for their effect on hydrolysis andHCl formation. Operating pressure is constrained to a feasible rangebounded by too little HCl production at high pressure andcrystallization of the brine if pressure is too low; this range is afunction of temperature and brine composition and is determined for eachtest. The temperature range evaluated is 300° to 500° C., but the testrange is also adapted based on test feedback in order to minimize theneeded heat source temperature. Brine composition has an effect onhydrolysis when comparing synthetic versus natural seawater; as aresult, pure CaCl₂ brine along with brine containing Mg, an expectedimpurity from seawater, are evaluated.

Hydrolytic Lime Product Testing. Precipitated solids that form in thehydrolysis reactor represent the material that can be used for seawatersoftening in a full-scale ocean CO₂ removal system. Favorable testconditions can be repeated and extended in time by injecting makeupsolution to achieve a quasi steady-state condition. These extended runscan be used to produce sufficient quantity of hydrolytic lime product(up to gram-size quantities) for detailed composition analysis and forsoftening effectiveness tests using synthetic seawater solutions. Theselatter tests can substantiate the stoichiometry of water softening usingbase material from brine hydrolysis. The target base material isCa(OH)₂, but could potentially include CaO, CaClOH, and unconvertedCaCl₂.

Hydrolysis data generated is used to validate a process simulation ofbrine hydrolysis in Aspen Plus; this unit operation model is, in turn,used to develop a complete process for efficiently extracting HCl andCa(OH)₂ products. The overall process separates HCl while recycling asmuch H₂O as feasible to avoid wasting energy on excessive watervaporization. Another design consideration for the process is a means torecycle sensible heat between the hot products and incoming brine.

In order to estimate the potential performance of a full-scalehydrolytic softening system for seawater, process modeling softwareAspen Plus is used alongside experimental data to produce a completeanalysis. But the specific energy consumption (i.e., kJ/kg CO₂) isdifficult to measure accurately an apparatus of this size. For this andother similar scenarios, chemical process models calibrated withmeasured experimental data is used to estimate the needed parameters.Estimates from techno-economic modeling are used to determine if a$100/ton levelized cost of CO₂ removal can be met.

Example 3 Supporting Data

In order to demonstrate the feasibility of applying hydrolytic softeningfor carbon dioxide removal from the ocean, laboratory experiments wereused to measure carbonate precipitation using Ca(OH)₂ as the softeningreagent, and to show the potential for the complete recovery of calciumthat is necessary to perpetuate a cycle of regeneration and reuse. Thesetests used three material streams to simulate the process of carbondioxide removal from the ocean, 1) artificial seawater, 2) softeningreagent solution, and 3) seed particles to serve as nucleation sites forcertain tests. Artificial seawater prepared according to ASTM D1141-98standards (dry solids supplied by Lake Products Company, Florissant,MO), was used as the seawater source. The softening reagent solution wasprepared by forming a saturated solution of Ca(OH)₂ in distilled water.Seed particles were composed of powdered CaCO₃ having a mean particlediameter of 48 μm. For tests that used seeds, they were added at anominal loading of 10 g/L, which provided approximately 0.6 m²/L ofnucleation surface area.

The test procedure consisted of adjusting the pH of each seawater sampleusing saturated Ca(OH)₂ solution, adding or withholding seed particles,and agitating the solution for 24 hours of contact time. At theconclusion of 24 hours, each seawater sample was filtered to removeprecipitates and the liquid was analyzed for alkalinity, calcium, andmagnesium. These data are summarized in Table 4.

TABLE 4 Summary Data for Seawater Softening Tests. Calculated RecoveredStarting Ending CO₂ Ca to Ca pH After Alkalinity, Ending EndingEquivalent Added as Seeds Ca(OH)₂ mg/L (as Ca, Mg, Removal, Ca(OH)₂,Test Present Addition CaCO₃) mg/L mg/L % mol/mol 1 No 8.21 138 593 13200.0 NA 2 No 8.91 91.4 566 1320 34.3 2.25 3 No 9.42 41.7 550 1320 73.61.87 4 No 9.84 42.1 531 1280 79.1 1.96 5 No 10.09 48.9 548 1240 79.81.54 6 No 10.17 41.7 581 1210 82.5 1.11 7 Yes 8.14 110 565 1270 20.7 NA8 Yes 8.86 105 570 1310 23.7 2.06 9 Yes 9.28 38.7 542 1310 73.4 2.04 10Yes 9.61 39.1 557 1310 81.9 1.56 11 Yes 9.82 41.7 560 1300 79.4 1.39 12Yes 10.06 41.2 589 1290 83.5 1.04

Test 1 in Table 4 represents the starting seawater solution and servedas the basis to calculate carbon dioxide removal and to determine theamount of recovered calcium. Carbon dioxide removal results are alsoplotted in FIG. 8 , and they indicate the potential for high efficiency,up to 73%, at adjusted pH values in the range of 9.0 to 9.5. Thecorresponding magnesium data in Table 4 confirm that carbon dioxideremoval within this pH range occurred before any significantco-precipitation of magnesium hydroxide.

Within the pH range of approximately 9.0 to 9.5, the calcium carbonatesaturation index (as calcite) approaches a maximum, but supersaturationof the unproductive and competitive precipitant magnesium hydroxide (asbrucite) does not occur. This preferred range of pH adjustment for oceancarbon dioxide removal is highlighted in FIG. 9 , which is a plot ofcalculated saturation index values for calcite and brucite.

One aspect of hydrolytic softening is to operate as a closed cyclewithout net input of calcium. To make cyclic operation possible, thecalcium used for pH adjustment as Ca(OH)₂ needs to be recovered as CaCO₃precipitate, and recycled using the process of brine hydrolysisregeneration. To confirm that cyclic operation is possible, theexperimental calcium recovery values from Table 4 are plotted in FIG. 10. Over the adjusted pH range of interest for seawater, 9.0 to 9.5, therecovery ratio has a value near two, meaning that nearly double thecalcium needed for cyclic operation could potentially be recovered. Thiscalcium recovery excess is an important operating characteristic sinceit will allow optimization of the precipitation stage in terms ofresidence time and circulation rate, and will ultimately lead to a lowercost of implementation.

Example 4. Economic Analysis of Hydrolytic Softening of Ocean Water forCarbon Dioxide Removal

The key advantage offered by the proposed technology is a reduction inthe energy cost required for ocean CO₂ removal. Energy cost savings areachieved by eliminating the need to concentrate brine prior to saltsplitting and by incorporating a unique thermochemical approach forsplitting. The impact of reduced energy consumption is manifested in theestimated performance metrics summarized in Table 5.

TABLE 5 Summary of performance metrics. Performance Metric TargetEstimated Value Levelized Cost of <$100/ton CO₂ $62/ton of net CO₂removed CO₂ Capture Second-Law Efficiency >10%  27% Embodied Emissions <5% 0.9% (as % of life cycle captured emissions)

The second-law efficiency determination was based on the minimum heat ofreaction for bicarbonate ion conversion to CO₂ (HCO₃ ⁻(aq) CO₂(g)+OH⁻(aq)), which is +66 kJ/mol CO₂ at reference conditions. Energyconsumption for hydrolytic softening was estimated to include two parts:the sensible energy needed to reach hydrolysis conditions (assumed to be400° C.) from the reference state and the reaction energy forhydrolysis. Sensible energy consumption was estimated to be +35 kJ/molCO₂ using a simple Aspen Plus model of the recuperative heating of aCaCl₂ brine from 20° to 400° C. with a 50° C. heat exchanger temperatureapproach limit. Hydrolysis energy was estimated to be +214 kJ/mol CO₂ bysetting a 60% thermal efficiency target for CaCl₂ hydrolysis, Reaction 5below, which has a theoretical reaction heat of +128 kJ/mol CO₂.Combined, these estimates result in a preliminary value of +249 kJ/molCO₂ for hydrolytic softening; this compared to the minimum energy forCO₂ removal at the reference state results in a second-law efficiencyestimate of 27%.CaCl₂+2H₂)→Ca(OH)₂+2HCl  (Reaction 5)

Embodied emissions were based on a nominal 1-million-ton CO₂ per yearcapture facility. An embodied emission factor of 14.6 g CO₂/kWhedeveloped for coal-fired power plants was used to estimate theseemissions. The factor was converted to a thermal basis of 3.8 gCO₂/kWhth (assuming 45% thermal efficiency and a capacity factor of 1)and scaled for the estimated system firing rate of 163 MWth. The resultwas an estimate of 5890 tons CO₂/yr of embodied emissions for thelifetime of the project (20 years); this value was 0.9% of the estimatednet CO₂ captured for this scenario, or 644,000 tons CO₂/year.

Techno-Economic Analysis

A preliminary cost model has been developed for a hydrolytic softeningprocess sized for the nominal removal of 1 million tons of CO₂ from theocean per year. It was estimated that with the reduced energyrequirements of this concept and its use of thermal energy, a cost of$62/ton CO₂, appears achievable for ocean CO₂ removal, significantlybelow a $100/ton performance target. FIG. 11 illustrates process flowvalues used for the techno-economic assessment. Table 6 illustrates thepreliminary techno-economic analysis for a hydrolytic ocean CO₂ removalsystem with 1 million tons CO₂ per year nominal capacity.

TABLE 6 Preliminary Techno-Economic Analysis for a Hydrolytic Ocean CO₂Removal System with 1 million tons CO₂ per year Nominal Capacity. ItemBasis Estimated Value Ocean Softening Reservoir $35/m³ of estimatedretention $110,000,000 Capital volume Brine Hydrolysis and Carbonate$840/kWth of estimated heat $137,000,000 Decomposition Capital inputrate System Ca(OH)₂ Consumption Assumed 1:1 molar ratio of 2.07 × 10¹⁰mol/yr Ca(OH)₂ to CO₂ gas captured System Energy Consumption Brinehydrolysis energy 163 MWth required for needed HCl rate assuming 60%thermal efficiency Annual Energy Cost Natural gas at $3.43/MMBtu$16,700,000/yr Nominal CO₂ Removed from Input specification 1,000,000ton CO₂/yr   the Ocean Annual CO₂ Emissions from Emissions factor of 227g 356,000 ton CO₂/yr Energy Consumption CO₂/kWth for natural gas NetAnnual CO₂ Capture Difference between capture 644,000 ton CO₂/yr andenergy emissions Levelized Cost of Net CO₂ Assumed a 20-year projectlife, $62/ton CO₂ Removal 7% annual discount rate, and zero end of lifevalue

Carbonate removal rate. Determination assumed an incoming concentrationof 200 ppm in seawater and a softened concentration of 50 ppm (bothvalues given as CaCO₃). This range is typical of conventional softenerperformance and results in a seawater throughput of nearly 1.6 millionm³/hr for a nominal 1 million tons of CO₂/year. The net CO₂ emissionswere somewhat lower in Table 6 to account for CO₂ associated with energyproduction.

Hydrated lime and HCl consumption. Determined from the stoichiometricratio of 1 mole Ca(OH)₂ and HCl to separate and release 1 mole of CO₂gas. In reality it is likely that closer to 2 moles of CaCO₃ willprecipitate per mole of added Ca(OH)₂ because of Ca(HCO₃)₂ existing inseawater, but the production of CO₂ gas will be constrained by HClavailability.

Brine hydrolysis energy consumption was estimated using a hydrolysisefficiency target of 60% and a reasonable sensible heat recuperationassumption of 3800 to 5700 kJ/kg CO₂. The energy basis for hydrolysiswas estimated to be +249 kJ/mol CaCl₂. These assumptions resulted in aplant heat input rate of 163 MWth for the 1-million-ton/year CO₂ removalrate.

Power cost was assumed to be dominated by the thermal energy for brinehydrolysis. Natural gas at $3.43/MMBtu and with a carbon intensity of227 g CO₂/kWth was the energy source. To account for the CO₂ releasedfrom gas consumption, these emissions were deducted from the plant'snominal 1-million-ton/year capacity and the resulting cost of CO₂removal was normalized on a net CO₂ removal basis to result in an annualcost of $16,700,000 or roughly $26 per ton of net CO₂.

Capital cost for solids regeneration. Capital for Steps 2 and 3 in FIG.11 was estimated by scaling the costs for a modern supercriticalcoal-fired power plant, an engineered system perceived to havesimilarities to the eventual brine hydrolysis process in terms ofoperating temperatures and pressures and that incorporates a similarvariety of unit operations (e.g., large-scale heat generation and heattransfer, emission control, solids collection and transport, and thelike). Assumed cost on an electrical output basis was $2100/kWe or$840/kWth on a thermal input basis.

Capital cost for ocean softening. Capital for Step 1 in FIG. 11 is basedon what are assumed to be existing analog structures to the floatingreservoir envisioned for softening, i.e., floating cages used forlarge-scale ocean-based aquaculture. The key cost driver for thesoftener infrastructure is based on the retention time needed for limemixing and settling of the precipitated carbonates. A typical retentiontime value of 2 hours was assumed; at the nominal seawater throughputthis sizing criteria resulted in a 3.1 million m³ enclosed volumecapacity. This volume is roughly one order of magnitude larger than thelargest aquaculture systems in use today, suggesting that an ocean CO₂removal system would require multiple units or the development of largersystems. Costs were assumed using $35/m³ of enclosed volume based on asurvey of aquaculture cage designs.

The terms and expressions that have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of the aspectsof the present invention. Thus, it should be understood that althoughthe present invention has been specifically disclosed by specificaspects and optional features, modification and variation of theconcepts herein disclosed may be resorted to by those of ordinary skillin the art, and that such modifications and variations are considered tobe within the scope of aspects of the present invention.

Exemplary Aspects

The following exemplary aspects are provided, the numbering of which isnot to be construed as designating levels of importance:

Aspect 1 provides a method of treating a metal carbonate salt, themethod comprising:

-   -   hydrolyzing a metal halide salt to form a hydrohalic acid and a        hydroxide salt of the metal in the metal halide salt, the metal        comprising an alkaline earth metal or an alkali metal; and    -   reacting the hydrohalic acid with the metal carbonate salt,        wherein the metal carbonate salt is a carbonate salt of the        alkaline earth metal or alkali metal, to form CO₂ and the metal        halide salt, wherein at least some of the metal halide salt        formed from the reacting of the hydrohalic acid with the metal        carbonate salt is recycled as at least some of the metal halide        salt in the hydrolyzing of the metal halide salt to form the        hydrohalic acid and the hydroxide salt.

Aspect 2 provides the method of Aspect 1, wherein the metal carbonatesalt is BeCO₃, MgCO₃, CaCO₃, SrCO₃, BaCO₃, RaCO₃, Li₂CO₃, Na₂CO₃, K₂CO₃,Rb₂CO₃, Cs₂CO₃, Fr₂CO₃, or a combination thereof.

Aspect 3 provides the method of any one of Aspects 1-2, wherein themetal carbonate salt is CaCO₃, MgCO₃, or a combination thereof.

Aspect 4 provides the method of any one of Aspects 1-3, wherein themetal carbonate salt is CaCO₃.

Aspect 5 provides the method of Aspect 4, wherein the CaCO₃ is producedfrom a CO₂-capture sorbent, is a CaCO₃ precipitate formed from watersoftening, is natural limestone, or a combination thereof.

Aspect 6 provides the method of any one of Aspects 1-5, wherein thealkaline earth metal or alkali metal is beryllium, magnesium, calcium,strontium, barium, radium, lithium, sodium, potassium, rubidium, cesium,francium, or a combination thereof.

Aspect 7 provides the method of any one of Aspects 1-6, wherein thealkaline earth metal or alkali metal is magnesium, calcium, or acombination thereof.

Aspect 8 provides the method of any one of Aspects 1-7, wherein thealkaline earth metal or alkali metal is calcium.

Aspect 9 provides the method of any one of Aspects 1-8, wherein themetal halide salt is a beryllium halide salt, a magnesium halide salt, acalcium halide salt, a strontium halide salt, a barium halide salt, aradium halide salt, a lithium halide salt, a sodium halide salt, apotassium halide salt, a rubidium halide salt, a cesium halide salt, afrancium halide salt, or a combination thereof.

Aspect 10 provides the method of any one of Aspects 1-9, wherein themetal halide salt is beryllium chloride, magnesium chloride, calciumchloride, strontium chloride, barium chloride, radium chloride, lithiumchloride, sodium chloride, potassium chloride, rubidium chloride, cesiumchloride, francium chloride, or a combination thereof.

Aspect 11 provides the method of any one of Aspects 1-10, wherein themetal halide salt is CaCl₂, MgCl₂, or a combination thereof.

Aspect 12 provides the method of any one of Aspects 1-11, wherein themetal halide salt is CaCl₂.

Aspect 13 provides the method of any one of Aspects 1-12, wherein thehydrohalic acid is HCl, HBr, HI, HF, or a combination thereof.

Aspect 14 provides the method of any one of Aspects 1-13, wherein thehydrohalic acid is HCl.

Aspect 15 provides the method of any one of Aspects 1-14, wherein thehydroxide salt is Be(OH)₂, Mg(OH)₂, Ca(OH)₂, Sr(OH)₂, Ba(OH)₂, Ra(OH)₂,LiOH, NaOH, KOH, RbOH, CsOH, FrOH, or a combination thereof.

Aspect 16 provides the method of any one of Aspects 1-15, wherein thehydroxide salt is Ca(OH)₂, Mg(OH)₂, or a combination thereof.

Aspect 17 provides the method of any one of Aspects 1-16, wherein thehydroxide salt is Ca(OH)₂.

Aspect 18 provides the method of any one of Aspects 1-17, wherein

-   -   the metal carbonate salt is CaCO₃,    -   the alkaline earth metal or alkali metal is calcium,    -   the metal halide salt is CaCl₂,    -   the hydrohalic acid is HCl, and    -   the hydroxide salt is Ca(OH)₂.

Aspect 19 provides the method of any one of Aspects 1-18, wherein thehydrolyzing of the metal halide salt is performed at a pressure of 0.1MPa −100 MPa.

Aspect 20 provides the method of any one of Aspects 1-19, wherein thehydrolyzing of the metal halide salt is performed at a pressure of 3 MPato 9 MPa.

Aspect 21 provides the method of any one of Aspects 1-20, wherein thehydrolyzing of the metal halide salt is performed at a pressure of 5-7MPa.

Aspect 22 provides the method of any one of Aspects 1-21, wherein thehydrolyzing of the metal halide salt is performed at a temperature ofroom temperature to 1000° C.

Aspect 23 provides the method of any one of Aspects 1-22, wherein thehydrolyzing of the metal halide salt is performed at a temperature of300° C. to 500° C.

Aspect 24 provides the method of any one of Aspects 1-23, wherein thehydrolyzing of the metal halide salt is performed at a temperature of350° C. to 450° C.

Aspect 25 provides the method of any one of Aspects 1-24, wherein thehydrolyzing of the metal halide salt produces the hydrohalic acid at amolar content of 0.01% to 10%.

Aspect 26 provides the method of any one of Aspects 1-25, wherein thehydrolyzing of the metal halide salt produces the hydrohalic acid at amolar content of 0.1% to 1%.

Aspect 27 provides the method of any one of Aspects 1-26, wherein thereacting of the hydrohalic acid with the metal carbonate salt isperformed at a pressure of 0.1 MPa −100 MPa.

Aspect 28 provides the method of any one of Aspects 1-27, wherein thereacting of the hydrohalic acid with the metal carbonate salt isperformed at a pressure of 3 MPa to 9 MPa.

Aspect 29 provides the method of any one of Aspects 1-28, wherein thereacting of the hydrohalic acid with the metal carbonate salt isperformed at a temperature of room temperature to 500° C.

Aspect 30 provides the method of any one of Aspects 1-29, wherein thereacting of the hydrohalic acid with the metal carbonate salt isperformed at a temperature of 350° C. to 450° C.

Aspect 31 provides the method of any one of Aspects 1-30, wherein themetal halide salt formed from the reacting of the hydrohalic acid withthe metal carbonate salt is 0.001 wt % to 100 wt % of the metal halidesalt used in the hydrolyzing of the metal halide salt to form thehydrohalic acid and the hydroxide salt.

Aspect 32 provides the method of any one of Aspects 1-31, wherein themetal halide salt formed from the reacting of the hydrohalic acid withthe metal carbonate salt is 80 wt % to 100 wt % of the metal halide saltused in the hydrolyzing of the metal halide salt to form the hydrohalicacid and the hydroxide salt.

Aspect 33 provides the method of any one of Aspects 1-32, wherein thehydrolyzing of the metal halide salt and the reacting of the hydrohalicacid with the metal carbonate salt is performed together in apressurized reactor.

Aspect 34 provides the method of any one of Aspects 1-33, furthercomprising reacting a used CO₂-capture sorbent with the hydroxide saltto provide the metal carbonate salt that is a carbonate salt of themetal in the metal halide salt.

Aspect 35 provides the method of Aspect 34, wherein the used CO₂-capturesorbent is a used hydroxide-based, ammonia-based, and/or amine-basedCO₂-capture sorbent.

Aspect 36 provides the method of any one of Aspects 34-35, wherein theused CO₂-capture sorbent is derived from sorption of CO₂ by ahydroxide-based, ammonia-based, and/or amine-based CO₂-capture sorbent.

Aspect 37 provides the method of any one of Aspects 34-36, wherein theCO₂-capture sorbent is a used hydroxide-based CO₂-capture sorbent.

Aspect 38 provides the method of any one of Aspects 34-37, wherein theused CO₂-capture sorbent is Ca(HCO₃)₂, Mg(HCO₃)₂, K₂CO₃, Na₂CO₃, or acombination thereof.

Aspect 39 provides the method of any one of Aspects 34-38, wherein thereacting of the used CO₂-capture sorbent with the hydroxide salt toprovide the metal carbonate salt is performed at a pressure of 0.01 MPato 10 MPa.

Aspect 40 provides the method of any one of Aspects 34-39, wherein thereacting of the used CO₂-capture sorbent with the hydroxide salt toprovide the metal carbonate salt is performed at a pressure of about0.05 MPa to 0.2 MPa.

Aspect 41 provides the method of any one of Aspects 34-40, wherein thereacting of the used CO₂-capture sorbent with the hydroxide salt toprovide the metal carbonate salt is performed at a temperature of roomtemperature to 350° C.

Aspect 42 provides the method of any one of Aspects 34-41, wherein thereacting of the used CO₂-capture sorbent with the hydroxide salt toprovide the metal carbonate salt is performed at a temperature of 50° C.to 150° C.

Aspect 43 provides the method of any one of Aspects 34-42, furthercomprising contacting a CO₂-capture sorbent with CO₂ to form the usedCO₂-capture sorbent.

Aspect 44 provides the method of any one of Aspects 34-43, furthercomprising contacting Ca(OH)₂, Mg(OH)₂, KOH, and/or NaOH with CO₂ toform the used CO₂-capture sorbent.

Aspect 45 provides the method of any one of Aspects 34-44, wherein thereacting of the used CO₂-capture sorbent with the hydroxide salt to formthe metal carbonate salt also forms an unused CO₂-capture sorbent.

Aspect 46 provides the method of Aspect 45, wherein the unusedCO₂-capture sorbent is Ca(OH)₂, Mg(OH)₂, KOH, and/or NaOH.

Aspect 47 provides the method of any one of Aspects 45-46, wherein theunused CO₂-capture sorbent is KOH and/or NaOH.

Aspect 48 provides the method of any one of Aspects 45-47, furthercomprising providing the unused CO₂-capture sorbent for CO₂ capture.

Aspect 49 provides the method of any one of Aspects 34-48, wherein atleast some of the hydroxide salt of the metal formed in the hydrolysisof the metal halide salt to form the hydrohalic acid and the hydroxidesalt is recycled as at least some of the hydroxide salt used in thereacting of the used CO₂-capture sorbent with the hydroxide salt.

Aspect 50 provides the method of Aspect 49, wherein the hydroxide saltof the metal formed in the hydrolysis of the metal halide salt is 0.001wt % to 100 wt % of the hydroxide salt used in the reacting of the usedCO₂-capture sorbent with the hydroxide salt.

Aspect 51 provides the method of any one of Aspects 49-50, wherein thehydroxide salt of the metal formed in the hydrolysis of the metal halidesalt is 80 wt % to 100 wt % of the hydroxide salt used in the reactingof the used CO₂-capture sorbent with the hydroxide salt.

Aspect 52 provides the method of any one of Aspects 1-51, furthercomprising reacting NaHCO₃, Mg(HCO₃)₂, Ca(HCO₃)₂, KHCO₃, or acombination thereof, with the hydroxide salt to provide the metalcarbonate salt that is a carbonate salt of the metal in the metal halidesalt.

Aspect 53 provides the method of Aspect 52, wherein the method is amethod of softening water.

Aspect 54 provides the method of any one of Aspects 1-53, furthercomprising reacting a bicarbonate salt from a natural water source,wherein the bicarbonate salt is NaHCO₃, Mg(HCO₃)₂, Ca(HCO₃)₂, KHCO₃, ora combination thereof, with the hydroxide salt to provide the metalcarbonate salt.

Aspect 55 provides the method of Aspect 54, wherein the natural watersource comprises salt water, ocean water, brackish water, fresh water, astream, a pond, a lake, a river, or a combination thereof.

Aspect 56 provides the method of any one of Aspects 54-55, wherein thebicarbonate salt is Ca(HCO₃)₂.

Aspect 57 provides the method of any one of Aspects 54-56, wherein atleast some of the hydroxide salt of the metal formed in the hydrolysisof the metal halide salt to form the hydrohalic acid and the hydroxidesalt is recycled as at least some of the hydroxide salt used in thereacting of the bicarbonate salt with the hydroxide salt.

Aspect 58 provides the method of Aspect 57, wherein the hydroxide saltof the metal formed in the hydrolysis of the metal halide salt is 0.001wt % to 100 wt % of the hydroxide salt used in the reacting of thebicarbonate salt with the hydroxide salt.

Aspect 59 provides the method of any one of Aspects 57-58, wherein thehydroxide salt of the metal formed in the hydrolysis of the metal halidesalt is 80 wt % to 100 wt % of the hydroxide salt used in the reactingof the bicarbonate salt with the hydroxide salt.

Aspect 60 provides a method of treating CaCO₃, the method comprising:

-   -   hydrolyzing CaCl₂ to form HCl and Ca(OH)₂; and    -   reacting the HCl with the CaCO₃, to form CO₂ and CaCl₂, wherein        at least some of the CaCl₂ formed from the reacting of the HCl        with the CaCO₃ is recycled as at least some of the CaCl₂ in the        hydrolyzing of the CaCl₂ to form the HCl and the Ca(OH)₂.

Aspect 61 provides the method of Aspect 60, further comprising reactinga used CO₂-capture sorbent with the Ca(OH)₂, to form the CaCO₃, whereinat least some of the Ca(OH)₂ formed in the hydrolysis of the CaCl₂ toform the HCl and the Ca(OH)₂ is recycled as at least some of the Ca(OH)₂used in the reacting of the used CO₂-capture sorbent with the Ca(OH)₂.

Aspect 62 provides the method of any one of Aspects 60-61, furthercomprising reacting Ca(HCO₃)₂ from a water source (e.g., ocean water)with the Ca(OH)₂, to form the CaCO₃, wherein at least some of theCa(OH)₂ formed in the hydrolysis of the CaCl₂ to form the HC1 and theCa(OH)₂ is recycled as at least some of the Ca(OH)₂ used in the reactingof the Ca(HCO₃)₂ with the Ca(OH)₂.

Aspect 63 provides a method of regenerating a used hydroxide-basedCO₂-capture sorbent, the method comprising:

-   -   hydrolyzing a metal halide salt to form a hydrohalic acid and a        hydroxide salt of the metal in the metal halide salt, the metal        comprising an alkaline earth metal or an alkali metal;

reacting the used hydroxide-based CO₂-capture sorbent with the hydroxidesalt, to form a carbonate salt of the metal in the metal halide salt;and

-   -   reacting the hydrohalic acid with the carbonate salt, to form        CO₂ and the metal halide salt, wherein at least some of the        metal halide salt formed from the reacting of the hydrohalic        acid with the carbonate salt is recycled as at least some of the        metal halide salt in the hydrolyzing of the metal halide salt to        form the hydrohalic acid and the hydroxide salt.

Aspect 64 provides a method of regenerating a used hydroxide-basedCO₂-capture sorbent, the method comprising:

-   -   hydrolyzing CaCl₂ to form HCl and Ca(OH)₂;    -   reacting the used hydroxide-based CO₂-capture sorbent with the        Ca(OH)₂, to form CaCO₃; and    -   reacting the HC1 with the CaCO₃, to form CO₂ and CaCl₂, wherein        at least some of the CaCl₂ formed from the reacting of the HCl        with the CaCO₃ is recycled as at least some of the CaCl₂ in the        hydrolyzing of the CaCl₂ to form the HCl and the Ca(OH)₂.

Aspect 65 provides a method of softening water, the method comprising:

-   -   hydrolyzing a metal halide salt to form a hydrohalic acid and a        hydroxide salt of the metal in the metal halide salt, the metal        comprising an alkaline earth metal or an alkali metal;    -   reacting a bicarbonate salt from a water source with the        hydroxide salt, to form a carbonate salt of the metal in the        metal halide salt; and    -   reacting the hydrohalic acid with the carbonate salt, to form        CO₂ and the metal halide salt, wherein at least some of the        metal halide salt formed from the reacting of the hydrohalic        acid with the carbonate salt is recycled as at least some of the        metal halide salt in the hydrolyzing of the metal halide salt to        form the hydrohalic acid and the hydroxide salt.

Aspect 66 provides a method of softening water, the method comprising:

-   -   hydrolyzing CaCl₂ to form HCl and Ca(OH)₂;    -   reacting Ca(HCO₃)₂ from a water source with the Ca(OH)₂, to form        CaCO₃; and reacting the HCl with the CaCO₃, to form CO₂ and        CaCl₂, wherein at least some of the CaCl₂ formed from the        reacting of the HCl with the CaCO₃ is recycled as at least some        of the CaCl₂ in the hydrolyzing of the CaCl₂ to form the HCl and        the Ca(OH)₂.

Aspect 67 provides the method of any one or any combination of Aspects1-66 optionally configured such that all elements or options recited areavailable to use or select from.

What is claimed is:
 1. A method of softening water from a water source,the method comprising: hydrolyzing a metal halide salt to form ahydrohalic acid and a hydroxide salt of the metal in the metal halidesalt, the metal comprising an alkaline earth metal or an alkali metal;reacting a bicarbonate salt in the water from the water source with atleast some of the hydroxide salt, to form a carbonate salt of the metalin the metal halide salt; and reacting the hydrohalic acid with thecarbonate salt, to form CO₂ and the metal halide salt, wherein at leastsome of the metal halide salt formed from the reacting of the hydrohalicacid with the carbonate salt is recycled as at least some of the metalhalide salt in the hydrolyzing of the metal halide salt to form thehydrohalic acid and the hydroxide salt.
 2. The method of claim 1,wherein the bicarbonate salt is NaHCO₃, Mg(HCO₃)₂, Ca(HCO₃)₂, KHCO₃, ora combination thereof.
 3. The method of claim 1, wherein the watersource is a natural water source.
 4. The method of 3, wherein the waterfrom the water source comprises salt water, ocean water, brackish water,fresh water, or a combination thereof, and wherein the natural watersource comprises an ocean, a stream, a pond, a lake, a river, or acombination thereof.
 5. The method of claim 1, wherein the bicarbonatesalt is Ca(HCO₃)₂.
 6. The method of claim 1, wherein the hydroxide saltof the metal formed in the hydrolysis of the metal halide salt is 80 wt% to 100 wt % of the hydroxide salt used in the reacting of thebicarbonate salt with the hydroxide salt.
 7. The method of claim 1,wherein the metal carbonate salt is BeCO₃, MgCO₃, CaCO₃, SrCO₃, BaCO₃,RaCO₃, Li₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃, Cs₂CO₃, Fr₂CO₃, or a combinationthereof.
 8. The method of claim 1, wherein the metal carbonate salt isCaCO₃.
 9. The method of claim 8, wherein the CaCO₃ is a CaCO₃precipitate formed from water softening, is natural limestone, or acombination thereof.
 10. The method of claim 1, wherein the metal halidesalt is CaCl₂.
 11. The method of claim 1, wherein the hydrohalic acid isHCl.
 12. The method of claim 1, wherein the hydroxide salt is Ca(OH)₂.13. The method of claim 1, wherein: the metal carbonate salt is CaCO₃,the metal halide salt is CaCl₂, the hydrohalic acid is HCl, and thehydroxide salt is Ca(OH)₂.
 14. The method of claim 1, wherein thehydrolyzing of the metal halide salt is performed at a pressure of 0.1MPa to 9 MPa.
 15. The method of claim 1, wherein the hydrolyzing of themetal halide salt is performed at a temperature of 300° C. to 500° C.16. The method of claim 1, wherein the reacting of the hydrohalic acidwith the metal carbonate salt is performed at a pressure of 0.1 MPa to 9MPa.
 17. The method of claim 1, wherein the reacting of the hydrohalicacid with the metal carbonate salt is performed at a temperature of roomtemperature to 500° C.
 18. The method of claim 1, wherein the metalhalide salt formed from the reacting of the hydrohalic acid with themetal carbonate salt is 80 wt % to 100 wt % of the metal halide saltused in the hydrolyzing of the metal halide salt to form the hydrohalicacid and the hydroxide salt.
 19. The method of claim 1, wherein thehydrolyzing of the metal halide salt and the reacting of the hydrohalicacid with the metal carbonate salt is performed together in apressurized reactor.
 20. A method of softening water from a watersource, the method comprising: hydrolyzing a metal halide salt to form ahydrohalic acid and a hydroxide salt of the metal in the metal halidesalt, the metal comprising an alkaline earth metal or an alkali metal;reacting a bicarbonate salt in the water from the water source with thehydroxide salt, to form a carbonate salt of the metal in the metalhalide salt; and reacting the hydrohalic acid with the carbonate salt,to form CO₂ and the metal halide salt, wherein at least some of themetal halide salt formed from the reacting of the hydrohalic acid withthe carbonate salt is recycled as at least some of the metal halide saltin the hydrolyzing of the metal halide salt to form the hydrohalic acidand the hydroxide salt.